The present invention relates generally to a vehicular transmission which comprises a continuously variable transmission that varies the speed change ratio of the transmission continuously and transmits the driving force output from the engine to the drive wheels of a vehicle.
1. Field of the Invention
Swash plate type transmissions, which utilize a fluid for speed change control, and V-belt type transmissions are well known as continuously variable transmissions incorporated in vehicular transmissions. Such a vehicular transmission incorporating a continuously variable transmission provides a high driving quality for a vehicle because the rotational speed of the engine is controlled to follow a target engine rotational speed which is determined to change the speed change ratio continuously so as to match the condition of the vehicle to the momentary expectation of the driver.
2. Description of the Related Art
As an example of such transmission, there is a transmission which is designed to eliminate engine idling for fuel efficiency. In this transmission, when the throttle of the engine is closed while the vehicle is driving, the fuel supply to the engine is terminated to decelerate the vehicle and to stop the engine when the vehicle is brought into a halt. Generally, such a transmission is controlled to achieve a LOW speed change ratio before the vehicle comes into a complete halt, so that the vehicle can be started smoothly after the restart of the engine.
However, such transmissions are known to experience an undesirable phenomenon that the response of the actual engine rotational speed to follow the target engine rotational speed tends to become slow as the speed of the vehicle decreases. It is difficult to maintain a certain rotational speed for the engine while the speed change ratio of the transmission is being adjusted to a LOW ratio. If the transmission or the control system of the transmission is arranged not to perform an idling elimination control for a condition where the speed change ratio does not decrease to a LOW ratio while the engine rotational speed has decreased to a predetermined rotational speed, then the fuel supply to the engine is started again when the engine rotational speed has decreased. If this situation is repeated, then a level of fuel efficiency originally desired for the transmission is not achieved.
The speed change ratio can be controlled in accordance to a control value which is achieved as the sum of a proportional term and an integral term, where the proportional term is a product of the deviation of the engine rotational speed from the target engine rotational speed multiplied by a gain for this proportional term, and the integral term is a cumulative sum of proportional terms multiplied by a gain for this integral term (for example, in a V-belt type continuously variable transmission, this control value corresponds to the magnitude of the thrust which acts to change the pitch radii of the pulleys for varying the speed change ratio). There is a method which makes the above mentioned gain for the proportional term larger, thereby making the engine rotational speed come close to the target engine rotational speed as quickly as possible while the vehicle is driving at a slow speed. However, this method is likely to be affected by a hunting which is caused by the engine rotational speed chasing the target engine rotational speed. As a result, this irregular engine speed deteriorates the riding quality of the vehicle.
To solve such a problem, the present invention provides a control system for a vehicular transmission which system is capable of bringing the speed change ratio to a LOW ratio with a certain engine rotational speed without impairing riding quality while the vehicle is decelerating.
In order to achieve this objective, the present invention provides a control system for a power transmission which is used for driving a vehicle. This power transmission comprises an engine and a continuously variable transmission which conveys the driving force with a speed change from the engine to wheels of the vehicle, and the control system comprises idling elimination control means and speed change control means. The idling elimination control means functions to stop the engine when the vehicle is brought into a halt, and the speed change control means calculates a target engine rotational speed in consideration of the driving condition of the vehicle and varies the speed change ratio of the continuously variable transmission such that the rotational speed of the engine will follow the target engine rotational speed. While the vehicle is driving with the throttle of the engine being closed, the control system terminates fuel supply to the engine and directs the speed change control means to vary the speed change ratio to a substantially LOW ratio. In this case, the speed change control means makes the rotational speed of the engine follow the target engine rotational speed at a faster speed than that applied when the throttle is open.
With this control system, while the vehicle is decelerated without fuel supply to the engine, the speed of the engine rotational speed to follow the target engine rotational speed is controlled to be faster than when the throttle is open, so the transmission is ensured to be shifted into a LOW ratio without any delay even while the vehicle is driving at a low speed. Preferably, after the speed change ratio is shifted quickly to a substantially LOW ratio by the speed change control means, the vehicle comes to a halt, and then while maintaining the state of non fuel supply to the engine, the control system directs the idling elimination control means to stop the engine.
Therefore, this control system is advantageous in fuel efficiency even in a transmission which is arranged not to perform idling elimination but to restart the fuel supply to the engine if the speed change ratio is not at a LOW ratio when the engine rotational speed has decreased to a predetermined rotational speed. The reason is that the speed change ratio is adjusted quickly to a LOW ratio as described above, so the actual frequency of the fuel supply restart decreases. Therefore, shocks which may occur in relation to the restart of the fuel supply are also less frequent, so the riding quality of the vehicle is improved.
It is preferable that the speed change control means control the speed change operation of the continuously variable transmission by generating a control value (for example, the shift thrust Ft of an embodiment described in the following section) which is a sum of a proportional term and an integral term. Here, the proportional term is determined on the basis of the deviation of the rotational speed of the engine from the target engine rotational speed, and the integral term is determined on the basis of the cumulative sum of the deviation. In this control, when the throttle is closed while the vehicle is driving, the integral term is set to a larger value than when the throttle is open. As the control value is adjusted in this way by increasing only the integral term without any increase of the proportional term, it is possible to make the engine rotational speed follow the target engine rotational speed with no occurrence of hunting, thus maintaining a good riding quality for the vehicle.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.